1. Field of the Invention
The invention relates, first, to a process for the electrolytic processing of printed circuit boards with drillings, which are taken through a treatment bath or to a processing station by conveyor means, in which there are means for reducing the thickness of a metal-ion-depleted region (diffusion zone), which are in contact with the printed circuit boards. The preferred area of use for the invention is galvanization, which will be discussed in more detail below. The invention can also be used in electrolytic etching.
2. Discussion of the Prior Art
The surface of the items to be processed or their diffusion layer becomes depleted of metal ions in a disadvantageous manner during processing, because the cathode draws more ions to it than usually come out of the surrounding treatment bath. This leads to a reduction in the acceptable specific current density, and thus means that in order to achieve such a metal coating having a particular thickness, a correspondingly long treatment time is needed.
In order to overcome this disadvantage, the so-called xe2x80x9chigh-speedxe2x80x9d process can be used, in which the electrolyte is conducted at great speed and in great quantity along the cathodic surface between this surface and the anode (see European Patent 0 142 010 and German Patent 35 25 183, for example). It is true that using this process, an improved, i.e. increased, current density of the metal coating on the cathodic item is achieved. However, the production, control and discharge of such an electrolytic current is relatively expensive. The creation of the flow channels necessary for this requires added constructive expenditures. In addition, the manner of conveying the items to be processed can be complicated. For these reasons, it has become known only for continuous items to be processed, such as strips or wires.
In the article xe2x80x9cElectrolytic High-Power Overgalvanization of Steel Strip through Boundary Layer Controlxe2x80x9d (xe2x80x9cElectrolytische Hocheistungoverzinkung von Stahlband durch mechanische Grenzschichtbeeinflussungxe2x80x9d) by D. Meuthen and D. Wolfhard in the journal Metal Surface 36 (Metaloberflxc3xa4che 36) (1982, pp. 70-75), a process for the electrolytic galvanization of steel strip is described, in which the strip surface to be galvanized is pressed against and moved relative to a nonwoven fabric, similarly to in the so-called tampon or brush galvanizing process. The coating electrolyte is rinsed through the nonwoven fabric onto the steel surface. Higher current density is achieved through this process during electrolytic zinc deposition.
A similar principle is also described in European Patent Application 0 210 072 for the electrolytic coating of small metal parts which are coated with noble metals, especially for use as plug-type connectors.
Furthermore, a process for the coating of surfaces is known from U.S. Pat. No. 3,706,650, in which non-conductive, fluid-storing, porous, compressible means are pressed against the surfaces to be coated and moved relative thereto, whereby the counter-electrode to the object to be coated is located in the interior of these means. The electrolytic fluid flows from the outside to the contact surfaces between the porous, compressible means and the surfaces to be coated. The porous, compressible means also contains hard non-conductive particles, which somewhat roughen the surfaces of the metal work piece to be coated.
In the three aforementioned documents, no reference is made to the flow through drill holes required in the manufacture of printed circuit boards, because in all cases the surfaces to be coated are either flat or merely curved.
In the German Document for Public Inspection 14 46 045, a process for galvanization with simultaneous electrolytic cleaning of metal surfaces and a device for implementing this process are disclosed. What is involved in this case is the coating of large drillings in steel with the help of an inner electrode, the outer areas of which are rubbed on the inner areas of the drillings. A device of this type is not suitable, however, for the electrolytic processing of printing circuit boards with drillings.
From DE-OS 36 03 856, a process and a device for galvanizing flat items such as printed circuit boards are known. The flat items are grasped and transported by a cathodically connected roller pair rotating at a relatively slow rotational speed. The electrolyte is applied to the item by an anodically connected roller pair, the surface of which can absorb fluid. A small distance is deliberately maintained between the surface of the item and the surface of the anodic rollers. The rotational speed of the anodic rollers is relatively high, in order to attain a correspondingly fast electrolytic movement along the surface of the item. In this way, an increase in current density is achieved, compared to conventional immersion bath galvanization. Thus rotating, insoluble roller pairs as anodes are described. The metal is supplied in soluble fashion via the electrolyte. The anodic roller pairs are not located below the level of the bath; therefore the electrolyte must continuously be applied to the galvanization location. The quantity of supplied electrolyte is limited, not least of all due to the close plastic screenings over the rollers. This also limits the possible galvanization current density. The plastic screenings are needed, however, in order to delay an undesired galvanization of the other, cathodically-connected roller pairs, which serve to transport the circuit boards.
Because the rotating anodic roller pairs do not touch the upper sides of the circuit boards, the diffusion layer located on the surface is not mechanically disturbed. However, the space between the anodic roller and the upper side of the circuit boards and thus the space between the pairs themselves is needed, to allow the galvanizing currents for the two sides of the circuit boards to be individually set. During galvanization of the conductor path image, this is always necessary, because the two sides of the circuit board, in practice, have uneven copper areas. For this reason, the anodes of the upper side of the circuit board are fed from one bath current rectifier and the anodes of the bottom side are fed from a different rectifier. Each rectifier can be individually set in respect to current.
Another disadvantage of the aforementioned invention is the very poor flow through fine holes in the printed circuit boards. On board sides located opposite to one another, electrolyte is applied in a small quantity in an almost pressure-free manner. This prevents the flow from going through the holes, resulting in inadequate galvanization of the hole walls even to the point of burns in the holes.
The invention is based on the problem of designing a generic process so as to achieve the desired reduction in thickness of the ion-depleted border layer on the anode surface or cathode surface (diffusion layer), and to thus attain a correspondingly increased current density of the galvanization current flowing from the electrolyte onto the anodic or cathodic item, while avoiding, however, the constructive and process-related expenses of the so-called xe2x80x9chigh-speedxe2x80x9d flow technique.
This problem is solved and this object attained, first, starting from the generic prior art, in that in the presence of an anode and cathodic items to be processed, or in the presence of anodic items to be processed and a cathode, the surface to be treated of the items is wiped continuously by machine and the electrolyte is conveyed by a component vertical to the plane of the items and is passed through the drillings or drill holes (referred to hereinafter for the sake of simplicity as xe2x80x9cdrill holesxe2x80x9d) of the items.
This wiping of the area or areas in question serves, in a simple, advantageous and industrially implementable manner, to counteract the disadvantageous depletion of metal ions in the diffusion layer. The diffusion layer is largely destroyed, and thus the ion-depleted zone on the surface or surfaces in question is completely or at least largely eliminated. The metal ions of the electrolyte can therefore make their way directly onto the surface of the items in question, or, during etching, can be removed therefrom. In respect to further advantages, reference is made to the discussions below of the various possible designs of the invention, in respect to both process and object. By means of the combination according to the invention of the aforementioned wiping and the metallization of the surface which this achieves with the metallization of the inner walls of the drill holes based on conveying the electrolyte through the drill holes, an adequate treatment of the inner wall of the drill holes is simultaneously achieved, and thus the metallization of the treatment items is achieved on all required areas (surfaces and drill holes) with satisfactory results in one work step and using relatively simple measures. In this connection, the aforementioned combination has the advantage that the wiping sequence destroys any surface tension existing on the fluid in the drill holes, making it even easier to metallize the drill hole inner wall. As the examples of this invention show, the structural means for wiping, on the one hand, and for conducting the electrolyte through the drill holes (so-called xe2x80x9cfloodingxe2x80x9d), on the other, can be provided together in a simple and space-saving manner, and in the preferred design of the invention are even combined. Here reference is made, for example, to the design according to FIG. 15. A further advantage is that with the aforementioned floodingxe2x80x94particularly when this is still done by supporting a pressing of the electrolyte, e.g., by means of a pump, into the drill holes and/or a drawing of the electrolyte out of the drill holes by means of a suction devicexe2x80x94any particles that might still be present in the drill holes can be extracted from the drill holes and transported away. This avoids the danger of particles trapped in the drill holes becoming embedded through metallization and of the drill hole in question becoming clogged, which leads to rejection of that particular circuit board.
Furthermore, in this connection, the danger is avoided of abrasive or chip particles still adhering to the edge of the drill holes being grasped by the wiping coatings and transported along the surface of the items to be processed, which can damage and even render unusable the delicate surface of the items to be processed.
A complementary attainment of the object is represented by a process in which an appropriate relative movement between the cathodic or anodic items to be processed on the one hand and an anode-side or cathode-side wiping device on the other band achieves the effect according to the invention.
In achieving this relative movement, the transport movement of the items to be processed may be used as an alternative when the items are taken by conveyor means through a treatment bath.
The possible process measures provided according to the invention for the attainment of the object may preferably be used for galvanization, but may also be used for electrolytic etching. The items (work pieces) to be processed thereby has an anodic function, i.e., it is anodically connected. The etched-away metal is precipitated on a counter-electrode (cathode). The counter-electrode can be the wiping device. Then, in a later work step, the precipitated metal is carried away and re-obtained. Electrolytic etching of this type is an alternative to purely chemical etching. During electrolytic etching, a simultaneous wiping of the anodes disrupts a border layer appearing there.
In order to simplify the presentation of the invention, the invention will be discussed below, and in the description of the examples related to the drawings, in reference to processes and devices for galvanization.
The movement of the coated anode relative to the item to be galvanized, whereby the coating bears against the item, largely destroys the diffusion layer (see above) and thus completely or largely eliminates the metal-ion-depleted region on the surface of the items to be processed.
The ions of the electrolyte can make their way directly through the coating of the anode to the surface of the item to be processed and metallize it. In this way, relatively high current densities can be achieved, and with good quality, particularly with an even strength of metal layer precipitated on the item surface, e.g., a copper layer. This is a significant advantage, which is especially important when it is not the entire area that is to be coated, but only the surface of conductor paths located on a circuit board of this type. A further significant advantage of the invention is that the distance between the anode and the cathodic item is still determined only by the thickness of the coating on the wiping device, whereby this thickness, however, can be relatively small. Due to the very small distance thus achieved between the anode and the cathode, different field line concentrations of the galvanization current flowing from the anode to the cathode practically do not exist, or at least exist to a considerably lesser extent than in arrangements in which a comparatively large distance exists between the anode and the cathode. In previously known arrangements with a relatively large distance between the anode and cathode, it was necessary, in order to avoid damaging effects such as increased metal precipitations on edge regions, e.g., the so-called xe2x80x9cdog-bonexe2x80x9d effect, to make appropriate expenditures, e.g., to provide xe2x80x9cblendings.xe2x80x9d This is especially true in the case of flat items to be processed, such as electronic printed circuit boards, which have a great many drillings. Here, considerable scatterings (known as drill hole scatterings) can result on the edge region of the boards and in the drill holes.
A significant advantage of the invention is that it achieves higher current density values, without the depletion of metal ions causing so-called xe2x80x9cburnsxe2x80x9d or the like of the metal to be deposited. Blendings or similar measures against excessive scatterings of the galvanizing current are not needed. A further significant advantage of the invention is that it is suitable for the automatic galvanization of items running continuously through a unit (perforated boards or the like). This will preferably occur in the case of items arranged and transported horizontally (see, for example, DE-OS 36 24 481, discussed below). However, the invention can be used not only with horizontal, but also with vertical or slanted runs, and such usage will be accompanied by the advantages discussed above, particularly the avoidance of damaging scatterings at high current densities. When it is possible for higher current densities to be applied, as they can be as a result of this invention, the transport speed also becomes faster or the treatment route shorter. In particular, it is not necessary to ensure a disturbingly high flow rate of the electrolyte.
With the invention, a wiping effect is automatically achieved on the entire surface to be treated, in particular, on both sides or surfaces of flat items. For the sake of completeness, it should be mentioned that in manual galvanization, the process referred to as tampon galvanizing is known, used particularly for the processing, repair or improvement of larger components which cannot be processed or can be processed only with great difficulty in a galvanizing unit. Typical examples of this are the improvement or galvanization of metal church roofs, large monuments and the like. For details, reference is made to the publications of RUBINSTEIN in the journal Galvanotechnik (xe2x80x9cGalvano-Technologyxe2x80x9d) No. 73 (1982), pp. 120 ff; No. 79 (1988) pp. 2876 ff and pp. 3263 ff. Such a tampon process, however, can be used only for the aforementioned special cases, not for the industrial production of printed circuit boards with drillings and the like.
In a preferred design form of the invention, not only the transport speed of the processed items, but also the speed of the coated anodic device itself is used to achieve an intensive wiping effect. Depending on requirements and design, a great variety of effects and wiping speeds can be achieved.
The aforementioned relative speed may be low, e.g., almost approaching zero. Furthermore, in implementing the process according to the invention, pressure can be exercised by the wiping device on the items to be processed. In the presence of an elastic coating on the wiping device, the coating can be flattened or pressed. Apart from the fact that irregularities in the thickness of the items can be smoothed out in this way, this helps to reinforce the effect according to the invention of disrupting the diffusion layer. This is especially true for the disclosed combination of features.
Further process measures promote the passage of the electrolyte through the drill holes and thus the metallization of the inner walls of the drill holes. Furthermore, they contribute to extracting from the drill holes particles or the like present therein.
The invention is also based on the object of creating an arrangement for the electrolytic processing of printed circuit boards with drillings, which are taken by conveyor means through a treatment bath, or conducted to a treatment station, whereby means for reducing the thickness of a metal-ion-depleted zone (diffusion layer) are provided, which are in contact with the printed circuit boards, which arrangement achieves flawless electrolytic processing, possible with simple means, of one or both surfaces of the printed circuit boards as well as of the drillings or drill holes (hereinafter called xe2x80x9cdrill holesxe2x80x9d) located therein. This is possible in particular with the process measures according to one or more of the process claims of the present invention.
To attain these objects and solve these problems, first of all, the arrangement has means for wiping the surface or surfaces of either cathodic items or anodic items, and also has means for conducting the electrolyte, in a flow roughly vertical to the plane of the items to be processed, through the drill holes of the latter (flooding means). This principle of the invention is to be realized in a structurally simple form, as will be seen in greater detail in the discussion which follows, particularly the discussion of the examples.
Wiping devices with the coating are an especially advantageous design form of the invention for mechanical wiping.
It is recommended that the coating be allowed to bear against the surface of the items to be processed with a certain pressure force. This is especially advantageous in processing conductor paths and drill holes, because the fluid in the drill holes has on its surfaces a certain surface tension that is disturbed by this wiping, thus clearing the way for ions, in order to form a metal layer (during galvanization) on the surfaces. Such a pressure force can, for example, be achieved through spring bearings of the wiping device, especially the aforementioned rollers.
Flat items are especially suitable for being conveyed between rollers. Rollers per se are known in their structure and their use in units for processing flat objects; however, they are known not in this form of wiping rollers according to the principle of the invention, but only as conveyor and guide rollers and as squeeze rolls for sealing. The rollers have the advantage of their simple and robust structure and insertion into a processing unit. In contrast to known conveyor rollers or pressure rollers, however, these wiping rollers with their coatings have the function of achieving the discussed mechanical wiping effect, i.e., their rotational speed deviates deliberately from the transport speed of the items to be processed which bear against them. At the same time, these rollers can also be designed as a counter-electrode to the object to be processed. The aforementioned deviation in speed can be achieved by means of the respective values of the aforementioned speeds and/or their respective directions. At this point, it should also be noted that the aforementioned speed deviation and/or relative speed may be low, almost zero.
Furthermore, the wiping rollers with their coating can mechanically favor the carrying away of particles. This is especially true when the rollers are pressed with a certain pressure force onto the surface of the items to be processed. In this way, disruptive layers on the cathode, such as gas bubbles suspended on the surface, can also be removed.
Further preferred design forms for reinforcing the metallization of the inner wall of drill holes consist of means and devices for conveying the electrolyte through the drill holes (flooding) with appropriate overpressure or underpressure.
Further advantages and features of the invention are found in the description which follows and in the accompanying drawings of possible designs according to the invention.